According to the prior art, methods and measuring apparatuses that are based on confocal scan systems or optical coherence tomography (OCT) have proved to be successful for determining distances and/or for tomographic imaging of ocular structures.
The great technological advantage of OCT is the decoupling of the depth resolution from the transversal resolution. In contrast to microscopy, it is possible to detect therewith the three-dimensional structure of the object to be examined. The purely reflective, and thus contactless, measurement enables generating microscopic images in vivo.
For OCT methods, coherent light is used with the aid of an interferometer for measuring distances and for imaging on reflective and scattering samples. On the human eye, due to the changes of the refractive index occurring on the optical boundary surface and due to volume scattering, the OCT methods deliver measurable signals when scanning in depth. Optical coherence tomography is a very sensitive and fast method for interferometric imaging, which is widely used in particular in the medical field and in basic research. OCT scans of ocular structures are frequently used in ophthalmology for diagnosis and therapy monitoring as well as for planning interventions and for selecting implants. An example for OCT-supported diagnosis is the use of OCT scans of the retina for determining the thickness of the retinal nerve fiber layer (RNFL) for diagnosing glaucoma and for monitoring the course of the disease.
For example, the basic principle of the OCT method described in the U.S. Pat. No. 5,321,501 is based on white light interferometry and compares the transmit time of a back-scattered sample signal (or measurement signal) with the reference signal by means of an interferometer (primarily a Michelson interferometer). Here, the arm having a known optical path length (=reference arm) is used as a reference for the measuring arm in which the sample is located. The interference of the signals from both arms forms a pattern from which the scattering amplitudes can be determined in dependence on the optical delay between the arms, and thus a depth-dependent scattering profile can be determined which, analogous to ultrasonic technology, is designated as A-scan. Rapid variations of the optical delay between measuring arm and reference arm can be implemented, for example, by means of fiber lines (EP 1 337 803 A1) or by means of so-called rapid-scanning optical delays (RSOD) (U.S. Pat. No. 6,654,127 B2). In multi-dimensional raster scans, the beam is then guided transversally in one or two directions, as a result of which a two-dimensional B-scan or a three-dimensional volume tomogram can be recorded. If the reference arm length is kept constant, a two-dimensional C-scan can be obtained during lateral scanning of the measuring beam in two directions.
Here, the arm having a known optical path length (=reference arm) is used as a reference for the measuring arm (also called sample arm). The interference of the signals from reference arm and sample arm results in an interference pattern from which the relative optical path length of scattering signals within an A-scan (depth signal) can be read out. In one-dimensional raster scans, the beam, analogous to ultrasonic technology, is then guided transversally in one or two directions, as a result of which a two-dimensional B-scan, a C-scan or a three-dimensional tomogram can be recorded. Usually, a C-scan is to be understood as a two-dimensional tomogram that has been obtained through two-dimensional scanning with a constant reference arm length in a time-domain OCT. However, this term shall be used hereinafter as a synonym for all scans that are based on two-dimensional scanning, thus also for volume scans. Here, the amplitude values of the individual A-scan are represented in linear or logarithmized grey scale values or false color values. Furthermore, it is known that through a comparison with B-scans, volume scans can be corrected with regard to inaccuracies caused by movements of the samples (U.S. Pat. No. 7,365,865). From A. H. Bachmann et al. it is further known according to [1] that through a phase-resolved measurement, in particular through Doppler signal evaluations, additional information about dynamic processes can be obtained and represented.
Recording of A-scans is usually carried out with 400 Hz to 400 kHz, in exceptional cases even in the MHz range. Ophthalmological OCT systems have typical sensitivities of from 80 dB to 110 dB. The wavelength used depends on the targeted scanning area and the absorption and scattering behavior of the tissue. Retinal OCTs operate in most cases in the range of from 700 nm to 1100 nm, while anterior chamber OCTs preferably use long-wave radiation, for example, of 1300 nm, which is absorbed in the vitreous. However, anterior chamber OCTs can also be implemented by switching over retinal OCTs (US 2007/0291277 A1).
The axial measurement resolution of the OCT method is determined through the so-called coherence length of the light source used, which is inversely proportional to the bandwidth of the utilized radiation and is typically between 3 μm and 30 μm (short-coherence interferometry). The lateral measurement resolution is determined by the cross-section of the measuring beam in the scanning area and is between 5 μm and 100 μm, preferred below 25 μm. Due to its particular suitability for examining optically transparent media, said method is widely used in ophthalmology.
In the case of the OCT methods used in ophthalmology, two different basic types have established themselves. For determining the measured values with the first type, the length of the reference arm is changed and the intensity of the interference is continuously measured without considering the spectrum. This method is designated as “time domain” method (U.S. Pat. No. 5,321,501 A). On the contrary, in the other method, the method designated as “frequency domain”, the spectrum is considered when determining the measured values, and the interference of the individual spectral components is recorded. Thus, on the one hand, this is referred to as signal in the time domain and, on the other, as signal in the frequency domain (FD-OCT).
The advantage of the “frequency domain” method is the simple and fast simultaneous measurement, wherein complete information about the depth can be determined without the need of movable parts. This increases stability and speed (U.S. Pat. No. 7,330,270 B2).
Furthermore, in the frequency domain OCT a distinction is made whether the spectral information is obtained by means of a spectrometer (“spectral domain OCT”, SD-OCT) or by means of swept-source OCT (SS-OCT).
A device for swept-source optical coherence domain reflectometry by means of which an entire eye can be measured in an A-scan is described in the still unpublished specification DE 10 2008 063 225.2. For this purpose, the device comprises a tunable laser light source with a defined spectral line width and a defined sweep tuning, and at least one receiver for the light scattered back from the sample. In this manner, in particular, a low-cost and efficient distance measurement over the entire length of the eye is implemented, since, despite typical eye movements of up to 1000 μm/s and with merely moderate requirements for the sweep rate of the laser light source, disturbing signal losses due to sample displacements during the distance measurements between surfaces of the cornea and the retina can be avoided.
The great advantage of the OCT methods, as already mentioned, is the contactless measurement and the generation of microscopic images and even three-dimensional structures of the object to be examined and in particular of the tissue to be examined in vivo. A possible error source that impairs the generation of accurate measured values and topograms are movements of the sample during the measuring process. It is known from the prior art that the adverse effects of sample movements can be reduced by using swept-source OCT (SS-OCT) or pulsed spectral domain OCT (SD-OCT).
For example, it is set forth by S. H. Yun et al. in [2] that the significant movement artifacts generated by movement of the sample and/or probe during the exposure time can be considerably reduced by a short illumination of the individual CCD pixels. For this, pulsed and tunable broadband light sources are used. Through a so-called “snap shot” illumination, axial profiles of a sample with greatly reduced movement artifacts can be generated. It has been found that using pulsed or tunable broadband light sources can indeed be an alternative to the use of expensive high-speed cameras in connection with “time domain” methods.
In addition to the movement of the sample and/or the probe, the measurement results can also be negatively influenced by unintentional movements of the scanner system. In particular those movement components that vary the optical path length from the measuring system to the sample and back disturb the interferences in the measuring system and thus the measurement results. These disturbing variations of the optical path are designated hereinafter as “axial modulation” by the scanner.
While the movements of the sample and/or the probe at speeds of a few mm/s, i.e., a few Hz, are slow and can be compensated in a relatively simple manner, this does not apply to the rather high-frequency, axial modulations of the scanner system.
According to the known prior art, the occurring axial modulations of scanner systems have not been compensated up to now, but are avoided or at least minimized by the use of high-quality scanners. For this purpose, normally, single-axis scanner with little mechanical deflections perpendicular to the rotational axis (torsion modes) are used. In order to avoid “fringe washout”, the recording times τ for an A-scan are reduced in known OCT systems to much less than 1/f, and/or scanner systems with axial modulation amplitudes of far below λ/2 are used. However, both variants are very cost-intensive.
An example for the use of such stable scanners is given in [4]. Here, these scanners are used for a decentered deflection of a measuring beam in order to implement very small, defined phase modulations of the optical path in the measuring arm and/or to implement Doppler shifts that allow a reconstruction of full or complex FD-OCT signals. Hereby, the usable modulations are limited due to the beginning fringe washout in the SD-OCT system used.
As particularly stable operating scanners, uniformly rotating polygon mirrors or oscillating galvanometer mirrors are used. Polygon mirrors are able to scan very fast and stable; however, they are limited to a given deflection pattern in a given direction. Moreover, they are very noisy and expensive. In contrast to that, galvanometer mirrors can implement different scan patterns, but they require very high control expenditure. Because of this, combinations of both systems are also often used as a scanner unit in ophthalmological apparatuses.
The most commonly used deflection systems in ophthalmological scanners, as described in the patent specification US 2008/231808 A1, comprise modern galvanometer scanners with an optical position detection system by means of which, via an electronic control unit, active control of the mirror movement including the damping of interferences (U.S. Pat. No. 5,999,302 A) is possible. An additional negative effect of these systems is that they are very complicated and expensive.
However, the use of simple scanner systems which, for example, can deflect the sample beams simultaneously in two directions entail the disadvantage that due to the increased number of bearings and the size of the suspensions, a sufficiently rigid design with tolerances that are much tighter than the wavelength, and thus lie in the sub-micrometer range, are extremely difficult so that minimizing occurring axial modulations is hardly possible.
Such a simple scanner system is described, for example, in the still unpublished patent specification DE 10 2009 041 995.0. The optical deflection unit is provided in particular for ophthalmological diagnosis and therapy apparatuses and comprises a deflection mirror, a position sensor and a control unit, which form a control circuit for minimizing the deviation of the actual positions detected by the position sensor from the targeted positions of the deflection mirror. A deflection mirror serves as an optical deflection unit, which deflection is movable by a contactless electromagnetic drive and oscillates about at least one rotational axis, and which, in the direction of the at least one rotational axis, is arranged between at least two bearings.
US 2009/225324 A1 describes a high-speed endoscope for optical coherence tomography that is based on a two-axis micro-mirror. Since the two-axis micro-mirror is usually moved with frequencies between 100 and 1000 Hz, accordingly, a fast OCT method is required for measured value acquisition. Used for this is a multi-function SD-OCT system that is capable of performing three-dimensional, intensity-sensitive and/or polarization-sensitive tomograms.
It is further known from the prior art that in the case of optical path length modulations of more than wavelength fractions per recorded A-scan in the OCT, major signal losses due to the so-called “fringe washout” are to be expected. S. H. Yun et al. document in [3] that the effect can go so far that in the case of optical path length variations of λ/2 per recorded scan, there is the risk of a complete signal loss because constructive and destructive interferences possibly average each other out.
Literature:
                [1] A. H. Bachmann, M. L. Villiger, C. Blatter, T. Lasser and R. A. Leitgeb, “Resonant        
Doppler flow imaging and optical vivisection of retinal blood vessels”, Vol. 15, No. 2/ OPTICS EXPRESS 408.                [2] S. H. Yun, G. J. Teamey, J. F. de Boer, and B. E. Bouma, “Pulsed-source and swept-source spectral-domain optical coherence tomography with reduced motion artifacts”, Vol. 12, No 23/ OPTICS EXPRESS 5614.        [3] S. H. Yun, G. J. de Boer, and B. E. Bouma, “Motion artifacts in optical coherence tomography with frequency-domain ranging”, Vol. 12, No. 13/OPTICS EXPRESS 2980.        [4] Lin An and Ruikang K. Wang, “Use of a scanner to modulate spatial interferograms for in vivo full-range Fourier-domain optical coherence tomography”, Vol. 32, No. 23/ OPTICS LETTERS.        